Adherent peritoneal cells from normal mice and mice infected with BCG were studied in vitro for their interaction with 15 different mouse tumor lines. The results were compared with assays for other features of macrophage activation, notably phagocytosis. A previously unsuspected degree of heterogeneity was revealed, both in terms of the patterns of "macrophage"-tumor interaction, and in terms of cell types present among adherent peritoneal cell populations and previously referred to simply as "macrophages." Specifically, four patterns of adherent peritoneal cell-tumor interaction were seen, ranging from inhibition of tumor proliferation to profound stimulation. The stimulatory effect of normal adherent peritoneal cells was much greater than that of such cells from BCG-treated mice, and was closely paralleled by the effect of 2-mercaptoethanol. Adherent peritoneal cells from BCG-treated mice displayed defective phagocytosis, due in part to the presence of increased numbers of nonphagocytic adherent cells. The latter were isolated and shown to have potent anti-tumor activity, which might have accounted for all that displayed by the whole adherent cell population. These cells may represent a heretofore undescribed subpopulation with certain properties of both macrophages and B lymphocytes.